When an object with parallel lines is positioned at an angle to a camera, the resulting image often appears distorted. This distortion is known as a keystone effect. The keystone effect is a distortion of the image dimensions, causing the tall object to look like a trapezoid which is the shape of an architectural keystone. The keystone effect may be corrected by shifting the lens of the image capturing device. The keystone effect is further exaggerated if the image capturing device is at a sharper angle to parallel lines in the subject.
Although usually discussed together, tilt and shift are two mechanisms that result in two completely different outcomes. The axis of a lens in a camera is normally perpendicular to an image sensor. Tilt results in a depth of field plane that is not parallel to the image sensor. Tilt alters depth of field and drastically alters the shape of the depth of field plane. Depth of field is the part of a still image or video that is “in focus”, as rendered by the optical elements of an imaging system. Tilt movements can be used for selective focus, such as enabling the floor of a room to be in focus in camera-leveled composition, or simulating a miniature scene. Moving the lens perpendicularly along the axis of the image causes shift, which can be observed easily by a camera operator when the camera operator previews an image.